MORTGAGE LOANS - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) restructuring	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Mortgage loans
Mortgage loans on real estate	$ 9,327,911		$ 9,327,911		$ 7,724,733
Mortgage call option period			10 years
Amount that would become due in remainder of 2019, if loans are called at their next call dates	54,400		$ 54,400
Amount that would become due in 2020 through 2024, if loans are called at their next call dates	850,900		850,900
Amount that would become due in 2025 through 2029, if loans are called at their next call dates	59,400		59,400
Commercial mortgage loans
Mortgage loans
Mortgage loans having participation feature	757,400		757,400		700,600
Income recognized on participating mortgage loans	3,800	$ 9,500	18,000	$ 22,000
Loans not subject to pooling and servicing agreement, non performing or restructured amount	3,200		$ 3,200
Number of troubled debt restructurings | restructuring			4
Allowance for mortgage loan credit losses	$ 4,056		$ 4,056		$ 1,296	$ 0
Minimum number of days for ceasing accrued interest			90 days
Minimum number of days past due to initiate foreclosure proceedings			90 days
Maximum number of days accrued interest on impaired loans			90 days
Maximum | Commercial mortgage loans
Mortgage loans
Loan-to-value ratio with participating interest (as a percent)	85.00%		85.00%